1. Field of the Invention
The present invention relates to a process for producing a polycarbonate, and more particularly to a process for efficiently producing a polycarbonate having excellent transparency, heat resistance, and water resistance.
2. DESCRIPTION OF THE BACKGROUND
One coventional method for producing a polycarbonate (hereinafter referred to as "PC"), is the interfacial method. This entails directly reacting an aromatic dihydroxy compound, such as bisphenol A, with phosgene. A second conventional method is the melt method which entails transesterifying an aromatic dihydroxy compound, such as bisphenol A, with a carbonic acid ester, such as diphenyl carbonate, both in a molten state.
The interfacial method suffers from several disadvantages. First, phosgene must be used, which is poisonous. Second, the production equipment used is corroded by chlorine-containing compounds, such as hydrochloric acid or sodium chloride, which are produced as by-products in the reaction. Third, impurities, such as sodium hydroxide, contaminate the resin and are difficult to separate therefrom. These impurities have deleterious effects on the physical properties of the polymer.
Alternatively, the melt method is advantageous in that PC can be produced thereby at a lower cost than by using the interfacial method. However, the melt method is disadvantageous due to the inevitable coloring of the resulting resin since the reaction usually continues for a long period of time at a temperature of 280.degree. to 310.degree. C.
To reduce the coloring in the melting method, various improvements have been proposed. Japanese Patent Publication No. 39972/1986 and Japanese Patent Application Laid-Open No. 223036/1988, for instance, disclose a method using a particular catalyst. Japanese Patent Application Laid-Open No. 151236/1986 and Japanese Patent Application Laid-Open No. 158719/1987 disclose methods wherein antioxidants are added in the latter stage of reaction. Moreover, Japanese Patent Application Laid Open No. 62522/1986 and Japanese Patent Application Laid-Open No. 153925 disclose improvements in processes, that is, employment of a twin-screw vent-type kneading extruder, and horizontal stirred polymerization tank, respectively. Further, Japanese Patent Application Laid-Open No. 175722/1990 discloses a process for controlling the content of a hydrolyzable chlorine compound in the monomer under a prescribed level. Such problems, however, have not yet been adequately solved, and a satisfactory PC has not yet been obtained.